The present invention relates to a steel sheet used in automobiles, household electric appliances, building materials, and the like, and to a method for manufacturing the same.
Industrial fields of automobiles and household electric appliances request for the reduction of production cost and the increase in productivity. Particularly in a press-forming process, the productivity increase has been promoted through the shortening of cycle time by speed increase and the extension of operation time. In that high level productivity, since the temperature increase in mold induces variations of press-forming conditions, there appear problems of generation of cracks and wrinkles, thus increasing in press-rejection rate.
As for the steel sheets for automobiles, occupied by press-forming steel sheets, there has been increasing the requirement to satisfy both the strength increase of steel sheets for improving safety and the work-saving in press-forming process including the reduction in the number of parts through integration of parts. To respond to the request, the steel sheets for press-forming are also required to have sufficient allowance in press-forming as well as the high formability.
To increase the press-formability and to increase the allowance, cold-rolled steel sheets using Ti-Nb-base very low C steels were developed, as disclosed in JP-B-7-62209, (the term xe2x80x9cJP-Bxe2x80x9d referred to herein signifies xe2x80x9cExamined Japanese Patent Publicationxe2x80x9d), and JP-B-47796, which sheets have already been supplied to automobile manufacturers. Along with the improvement of material qualities, however, the forming conditions of the manufacturers have become stricter than ever. As a result, under recent press-conditions, steel sheets of the above-described Tixe2x80x94Nb-base very low C steels give a problem of generation of press-rejection rate. With high strength steel sheets, also the frequency of press-rejection increases along with the widening of application components of that kind of steels.
In addition, the high strength galvanized steel sheets which undergo press-forming are requested to have deep-drawing performance and to have non-aging property to suppress generation of stretcher-strains. In the past, to improve the deep-drawing performance and the non-aging property, there were developed high strength steel sheets based on IF steels in which the contents of C and Mn are minimized, and Ti, Nb, and the like are added to fix harmful C and N as carbo-nitrides. The IF steels, however, have a problem of high sensitivity to the secondary working brittleness. Furthermore, since the grain boundary strength relatively decreases with the increase in the strength of the steel sheets, the secondary working brittleness likely occurs. Accordingly, the development of high strength steel sheets having excellent deep-drawing performance should emphasize the improvement of resistance to secondary working brittleness as a critical issue. There are several technologies to increase the resistance to secondary working brittleness while maintaining the characteristics almost equal with those of IF steels, as disclosed in JP-B-61-32375, JP-A-5-112845, (the term xe2x80x9cJP-Axe2x80x9d referred to herein signifies xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d), JP-A-5-70836, and JP-A-2-175837.
However, the steels of JP-B-61-32375 and JP-A-5-112845 increase the resistance to secondary working brittleness by leaving solid solution C therein, so that there is a problem of aging when the steels are allowed to stand in a relatively high ambient temperature, such as in summer, for a long period. The steels of JP-A-5-70836 increase the resistance to secondary working brittleness by the addition of B. Boron, however, segregates in grain boundaries to suppress the crystal rotation during cold-working, which hinders the development texture favorable in attaining high r value, and degrades the deep-drawing performance. The steels of JP-A-2-175837 increase the resistance to secondary working brittleness owing to the addition of Nb to bring the grain boundary shape in a saw-teeth shape, thus making grain boundary fracture difficult. Those types of characteristics, however, make the working difficult.
As for the press-formability of cold-rolled steel sheets, investigations have been conducted mainly from the standpoint of deep-drawing performance and of stretchability. Regarding the deep-drawing performance, increase in r value is focused on, as described in JP-A-5-58784 and JP-A-8-92656. When, however, the cold-rolled steel sheets described in JP-A-5-78784 and JP-A-8-92656 are applied to side panels which are formed mainly for stretching, the punch-shoulder portion where a flat deformation stretch forming is conducted may induce fracture owing to insufficient propagation of strain. To that type of fracture occurred during that kind of stretch-forming, no appropriate action can be given because the increased strength of the materials does not allow to give evaluation by the total elongation and the n value, which are applicable in conventional mild materials.
It is an object of the present invention to provide a steel sheet for press-forming, having large forming allowance during press-forming and giving reduced press-rejection rate, thus improving the productivity, and to provide a method for manufacturing thereof.
To attain the object, the present invention provides a steel sheet which consists essentially of: a ferritic phase having ferritic grains of 10 or more grain size number and ferritic grain boundaries; and at least one kind of precipitate selected from the group consisting of Nb-base precipitate and Ti-base precipitate, being included in the ferritic phase. Each of the ferritic grains has a low density region with a low precipitate density in the vicinity of grain boundary. The low-density region has a precipitate density of 60% or less to the precipitate density at center part of the ferritic grain.
The low density region preferably exists in a range of from 0.2 to 2.4 xcexcm distant from the ferrite grain boundary.
The steel sheet preferably has a BH value of not more than 10 MPa.
The steel sheet preferably consists essentially of 0.002 to 0.02% C, 1% or less Si, 3% or less Mn, 0.1% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.007% or less N, at least one element selected from the group consisting of 0.01 to 0.4% Nb and 0.005 to 0.3% Ti, by mass %, and balance of substantially Fe. The C content is more preferably from 0.005 to 0.01%. The Nb content is more preferably from 0.04 to 0.14%. The Nb content is most preferably from 0.07 to 0.14%. The Ti content is more preferably from 0.005 to 0.05%.
The steel sheet preferably consists essentially of 0.002 to 0.02% C, 1% or less Si, 3% or less Mn, 0.1% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.007% or less N, 0.002% or less B, at least one element selected from the group consisting of 0.01 to 0.4% Nb and 0.005 to 0.3% Ti, by mass %, and balance of substantially Fe. The B content is more preferably 0.001% or less.
A method for manufacturing the steel sheet comprises the steps of: hot-rolling a slab to prepare a hot-rolled steel sheet; cooling the hot-rolled steel sheet to a temperatures of 750xc2x0 C. or less at cooling speeds of 10xc2x0 C./sec or more; coiling the cooled hot-rolled steel sheet; cold-rolling the coiled hot-rolled steel sheet to prepare a cold-rolled steel sheet; and annealing the cold-rolled steel sheet.
The slab consists essentially of 0.002 to 0.02% C, 1% or less Si, 3% or less Mn, 0.1% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.007% or less N, at least one element selected from the group consisting of 0.01 to 0.4% Nb and 0.005 to 0.3% Ti, by mass %, and balance of substantially Fe.
The slab preferably consists essentially of: 0.002 to 0.02% C, 1% or less Si, 3% or less Mn, 0.1% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.007% or less N, 0.002% or less B, at least one element selected from the group consisting of 0.01 to 0.4% Nb and 0.005 to 0.3% Ti, by mass %, and balance of substantially Fe.
The ferritic grains of the coiled hot-rolled steel sheet preferably have 11.2 or more grain size number.
The step of coiling the hot-rolled steel sheet is preferably carried out at coiling temperatures of from 500 to 700xc2x0 C.
The step of cold-rolling the hot-rolled steel sheet is preferably carried out at least 85% of cold draft percentage.
The step of annealing the cold-rolled steel sheet is preferably carried out by continuous annealing at temperatures of from 900xc2x0 C. to recrystallization temperature.
Furthermore, it is another object of the present invention to provide a method for manufacturing a high strength cold-rolled steel sheet and a high strength zinc-base coated steel sheet, which have surface quality, non-aging property, and workability applicable to outer body sheets of automobiles, and which have excellent resistance to secondary working brittleness.
To attain the object, the present invention provides a steel sheet which consists essentially of: 0.004 to 0.02% C, 1.0% or less Si, 0.7 to 3.0% Mn, 0.02 to 0.15% P, 0.02% or less S, 0.01 to 0.1% Al, 0.004% or less N, 0.2% or less Nb, by mass %, and balance of substantially Fe; the Nb content satisfying a formula of
(12/93)xc3x97Nb*/Cxe2x89xa71.0 
where, Nb*=Nbxe2x88x92(93/14)xc3x97N, and
where, C, N, and Nb designate content of respective elements, (mass %); and yield strength and average grain size of the ferritic grains satisfying a formula of
YPxe2x89xa6xe2x88x92120xc3x97d+1280 
Where, YP designates yield strength [MPa], and d designates average size of ferritic grains [xcexcm].
The above-described steel sheet preferably has an n value determined by 10% or lower deformation in a uniaxial tensile test satisfies a formula of
n valuexe2x89xa7xe2x88x920.00029xc3x97TS+0.313 
where, TS designates tensile strength [MPa].
The C content is preferably from 0.005 to 0.008%. The Nb content is more preferably from 0.08 to 0.14%. The steel sheet preferably further contains 0.05% or less Ti. The steel sheet preferably further contains 0.002% or less B. The steel sheet preferably further contains at least one element selected from the group consisting of 1.0% or less Cr, 1.0% of less Mo, 1.0% or less Ni, and 1.0% or less Cu.
The steel sheet preferably has a zinc-base coating thereon.
A method for manufacturing steel sheet comprises the steps of: hot-rolling a slab at finish temperatures of Ar3 transformation point or above; coiling the hot-rolled steel sheet at temperatures of from 500 to 700xc2x0 C.; cold-rolling the coiled hot-rolled steel sheet; and annealing the cold-rolled steel sheet.
The slab consists essentially of 0.004 to 0.02% C, 1.0% or less Si, 0.7 to 3.0% Mn, 0.02 to 0.15% P, 0.02% or less S, 0.01 to 0.1% Al, 0.004% or less N, 0.035 to 0.2% Nb, by mass %, and balance of substantially Fe.
The method for manufacturing steel sheet preferably further contains a step for applying zinc-base coating on the steel sheet after annealed.
The slab preferably further contains 0.05% or less Ti.
The slab preferably further contains 0.002% or less B.
Furthermore, the present invention provides a steel sheet which consists essentially of: 0.0040 to 0.02% C, 1.0% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.07% P, 0.02% or less S, 0.01 to 0.1% Al, 0.004% or less N, 0.15% or less Nb, by mass %, and balance of substantially Fe; the Nb content satisfying a formula of
(12/93)xc3x97Nb*/Cxe2x89xa71.2 
where, Nb*=Nbxe2x88x92(93/14)xc3x97N, and
where, C, N, and Nb designate content of respective elements, (mass %); and yield strength and average grain size of the ferritic grains satisfying a formula of
YPxe2x89xa6xe2x88x9260xc3x97d+770 
Where, YP designates yield strength [MPa], and d designates average size of ferritic grains [xcexcm].
The C content is more preferably from 0.005 to 0.008%. The Nb content is more preferable from 0.08 to 0.14%.
The steel sheet preferably has an n value determined by 10% or lower deformation in a uniaxial tensile test is 0.21 or more.
The steel sheet preferably further contains 0.05% or less Ti. The steel sheet preferably further containing at least one element selected from the group consisting of 1.0% or less Cr, 1.0% of less Mo, 1.0% or less Ni, 1.0% or less Cu.
The steel sheet preferably has a zinc-base coating thereon.
A method for manufacturing steel sheet comprises the steps of: hot-rolling a slab consisting essentially of 0.004 to 0.02% C, 1.0% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.07% P, 0.02% or less S, 0.01 to 0.1% Al, 0.004% or less N, 0.035 to 0.15% Nb, by mass %, and balance of substantially Fe, at finish temperatures of Ar3 transformation point or above; coiling the hot-rolled steel sheet at temperatures of from 500 to 700xc2x0 C.; cold-rolling the coiled hot-rolled steel sheet; and annealing the cold-rolled steel sheet.